Add pause and play button

[oakes]

It would be helpful to have a button for pausing and playing the game. This would help analyze the state at a particular moment in time, and also reduce distraction while coding. The main questions are, (1) how to implement it on a technical level (does libGDX allow you to stop the game loop?), and (2) how to avoid UI clutter (there are already six buttons at the top left of the game screen).